1. Technical Field
This invention relates generally to an active suspension system, and more specifically, to an apparatus and method for an active vehicle suspension system incorporating frequency shaping characteristics to reduce impact harshness.
2. Discussion of the Related Art
Active suspension systems which exhibit suspension control over a wide bandwidth of disturbance frequencies are known to those skilled in the art. These prior art active suspension systems generally exhibit a substantially harsher impact characteristic than their original passive counterpart systems. Specifically what this means is that when a machine incorporating the suspension system, such as a vehicle, encounters a sharp disturbance, the forces transmitted to the machine are larger and exhibit higher frequency content than would be transmitted to the machine if a passive system were incorporated. Therefore, there is a trade off between the known advantages of the active system, such as higher stability and handling in a vehicle, and the reduced impact harshness of a passive system.
One method known in the art to reduce the impact harshness of an active suspension system of a vehicle is to reduce the hub or wheel damping. Unfortunately, this generally leads to unacceptable wheel damping well before a harshness characteristic comparable to the passive system can be achieved. This relationship is shown in FIG. 1 for a particular active and passive suspension system. The horizontal axis of FIG. 1 represents the hub damping as an undimensioned relationship of the time the wheel takes to stop oscillating after it encounters a road irregularity. As can be seen, the higher the hub damping the better wheel control is available, and thus the better the handling of the vehicle. As the hub damping decreases, the wheel bounce increases and the control and handling of the vehicle becomes more difficult and possibly unsafe. The vertical axis represents the peak-to-peak body load as the amount of force being transmitted to the vehicle from the suspension system. An increase in body load translates to an increase in harshness upon impact of a road irregularity.
Solid line A represents the relationship between body load and hub damping for a specific vehicle having an active suspension system. Dashed line B represents the relationship between hub damping and body load for a particular vehicle having a passive suspension system. As can be seen, a reduction in hub damping reduces the harshness of a particular road disturbance. However, it is apparent from curves A and B that the hub damping cannot be reduced enough in the active system to achieve comparable impact harshness to that of the passive system before getting into a region of unreasonable wheel bounce. Unreasonable wheel bounce being defined as a factor of driver discomfort and/or loss of vehicle control. Therefore, this method of reducing the hub damping has met with limited success.
Another method to attempt to reduce the impact harshness of an active suspension system is the use of soft isolator bushings, as shown in FIG. 2. In that figure, two devices for inducing wheel damping are shown between a sprung mass 10 and an unsprung mass 12. For visual purposes the sprung mass could be the body of a particular vehicle and the unsprung mass could be one of the vehicles wheels. The two wheel damping devices are a variable control damper 16 and an active actuator 18. Damper 16 can either be incorporated into a passive or active system and actuator 18 is generally used in an active system. A spring 14 is generally used in both a passive or active system to further increase suspension control. Two soft isolator bushings shown generally at 20 have been incorporated in conjunction with both damper 16 and active actuator 18 between the sprung mass 10 and the unsprung mass 12.
Soft isolator bushings 20 help reduce impact harshness due to smaller impacts, as well as high frequency (noise) transmission of road disturbances which would be too small to be damped by wheel damping devices 16 and 18. However, to maintain proper wheel damping in a suspension system incorporating soft isolator bushings 20, higher peak damping forces are required. As described above for FIG. 1, higher damping forces results in increased transmission of larger single road disturbances, which results in increased impact harshness. In addition, a soft isolator bushing requires substantial bushing travel to prevent the bushing from encountering the bushing travel limits at too low of a load. Further, an isolator bushing creates a bushing induced phase lag in the damper due to the up and down movement of the damper shifted in time from the up and down movement of the rest of the suspension system. As a practical matter, however, isolator bushings are required for both passive and active suspension systems to reduce noise disturbances to the vehicle chassis, and the performance characteristics which result from the isolator bushings for both systems are similar.
What is needed then is an apparatus and method for reducing impact harshness of an active suspension system and further, compensating for the undesirable effects of an isolator bushing and other system dynamics, including bushing induced phase lag, while allowing for independent control of hub damping and high frequency transmission characteristics. It is therefore an object of the present invention to provide such a method and apparatus.